Electronic systems commonly utilize electronic signals for processing information or performing specified functions. Electronic signals are time varying voltage or current values that occur at various specified portions of the electronic system. For example, a particular circuit node may experience a particular change in voltage over time. Moreover, such a change in the voltage may take on recurring values over time in a regular manner (i.e. the voltage on the node is periodic). Typically, time varying voltages and currents are compared, added, subtracted. multiplied, or otherwise processed to perform various functions corresponding to the particular type of electronic system. In order to more efficiently carry out such processing, many electronic systems utilize circuits which generate an electronic signal with a constant period. Such circuits are known as oscillator circuits and are found in many types of electronic systems.
Generally, an oscillator circuit is a device for periodically transferring electrical energy between components. FIG. 1 illustrates a closed loop architecture commonly employed for oscillator circuits. The oscillator circuit of FIG. 1 is comprised of a feed forward stage and a feedback stage. Amplifiers are commonly used in the feed forward stage to produce the periodic electronic output signal Vo so that the oscillations will continue indefinitely without damping out. The feedback stage receives the periodic electronic output signal Vo and generates a feedback control signal Vf which is either added, subtracted or in some other way combined with the system input signal Vs to control the periodic electronic input signal Vin at the feed forward stage. In order for the closed loop architecture of FIG. 1 to achieve steady state oscillation, the amplifier A(j.omega.) must provide sufficient gain, and the feedback stage F(j.omega.) must shift the phase of the output signal a sufficient amount such that the feedback control signal is in phase with the input signal. The mathematical expression of these requirements can be established by following the signal flow around the feedback loop. The gain of the amplifier can be written as EQU Vo=VinA(j.omega.).
The gain around the loop can be established as EQU Vf=Vo F(j.omega.)=VinA(j.omega.)F(j.omega.),
and the transfer function of the system is EQU Vf/Vin=A(j.omega.)F(j.omega.).
Therefore, the magnitude and phase conditions for oscillation can be shown as: EQU .vertline.Vf/ Vin.vertline.=.vertline.A(j.omega.).vertline..vertline.F(j.omega.).vertlin e..gtoreq.1.0.phi..sub.A +.phi..sub.B =0.degree.
where .phi..sub.A and .phi..sub.B are the phase shifts associated with the amplifier and feedback network respectively.
Oscillator circuits according to the architecture of FIG. 1 can be implemented in a variety of ways. The underlying characteristic common to each implementation is the idea that electric energy is transferred between various parts of the system during oscillation cycles. For example, some oscillator circuits charge or discharge an inductor or capacitor during the alternate phases of the oscillation cycle. The use of inductors and/or capacitors for storing and discharging energy is an example of an oscillation system which transfers electromagnetic energy between two or more devices capable of storing electromagnetic energy. An example of a well-known oscillation system that utilizes the transformation of electromagnetic energy is the Wien-Bridge Oscillator shown in FIG. 2. The feedback stage of the Wien-Bridge Oscillator uses a resistor and capacitor network for alternately storing and discharging electromagnetic energy according to principles well-known by those skilled in the art.
Other well known oscillator circuits employ energy storage elements that store mechanical energy during the course of an oscillation cycle. A crystal oscillator is a typical example. In a crystal oscillator, electromagnetic energy is transferred into mechanical energy by applying an electric signal across the crystal structure, thereby causing the crystal lattice to change its physical orientation. Such a change in orientation is achieved by the absorption of electromagnetic energy. For example, in a quartz crystal oscillator, a voltage applied across the crystal will cause the crystal to move sideways internally in a thickness shear movement. Moreover, a quartz crystal can be modeled as a damped LC circuit, and will have a resonant frequency corresponding to the physical properties of the crystal structure. Examples of two oscillator circuits that utilize the mechanical energy storage properties of quartz crystals are shown in FIG. 3 and FIG. 4. The oscillator circuit of FIG. 3 utilizes a feed forward stage comprised of a amplifier 301. The feedback stage utilizes a quartz crystal 302 in a series configuration. In this configuration electrical energy at the resonant frequency of the quartz crystal 302 is transferred into mechanical energy in the crystal structure, and then back into electrical energy at the input of the amplifier. FIG. 4, on the other hand, shows a shunt oscillator circuit with feedback stage comprised of a series capacitor 403 and a shunt quartz crystal 402. The shunt oscillator circuit of FIG. 4 includes a 90.degree. phase lag network using a shunt impedance 406 and the amplifier's source resistance, illustrated by resistor 405, to provide the phase lag. The lag network is required to compensate for the 90.degree. phase shift introduced by capacitor 403 to bring the total phase around the loop to 0.degree. as required for oscillation. In both series and shunt configurations of FIG. 3 and FIG. 4 respectively, the crystal is being used to transfer electromagnetic energy into mechanical energy during alternate phases of the oscillation cycle.
However, electromagnetic and mechanical storage elements share a common problem. In an effort to reduce costs, electronic system designers strive to integrate various components onto a single semiconductor integrated circuit while utilizing a minimal amount of semiconductor area. Unfortunately electromagnetic storage elements such as capacitors and inductors typically occupy large areas of semiconductor area and are, therefore, often provided by external discrete components that occupy large areas on the printed circuit board and correspondingly increasing system costs. Mechanical storage elements also are traditionally provided as external discrete components which also occupy large areas on the printed circuit board and correspondingly increase system costs. Therefore, there is a need to find other solutions for enabling oscillation circuits which can be fully integrated onto a single semiconductor integrated circuit.